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Abstract:

Various embodiments of optical fiber designs and fabrication processes
for ultra small core fibers (USCF) are disclosed. In some embodiments,
the USCF includes a core that is at least partially surrounded by a
region comprising first features. The USCF further includes a second
region at least partially surrounding the first region. The second region
includes second features. In an embodiment, the first features are
smaller than the second features, and the second features have a filling
fraction greater than about 90 percent. The first features and/or the
second features may include air holes. Embodiments of the USCF may
provide dispersion tailoring. Embodiments of the USCF may be used with
nonlinear optical devices configured to provide, for example, a frequency
comb or a supercontinuum.

Claims:

1. (canceled)

2. An optical fiber capable of propagating light having a wavelength, the
optical fiber comprising: a core having a diameter less than about 4
μm; a first region at least partially surrounding the core, the first
region comprising a plurality of first features collectively having a
first filling factor in the first region that is less than about 90
percent; and a second region at least partially surrounding the first
region, the second region comprising a plurality of second features
collectively having a second filling factor in the second region that is
greater than about 90 percent, wherein the optical fiber has a dispersion
tailored to provide a spectral bandwidth of at least about 50 nm in a
nonlinear optical device.

3. The optical fiber of claim 2, wherein the core diameter is in a range
from about 1 μm to about 4 μm.

4. The optical fiber of claim 2, wherein the first filling factor is
greater than about 50%.

5. The optical fiber of claim 2, wherein the first filling factor is less
than the second filling factor.

6. The optical fiber of claim 2, further comprising: an outer layer
surrounding the second region; and a plurality of webs mechanically
coupling the first region and the outer layer such that the second region
is disposed therebetween.

7. The optical fiber of claim 6, wherein the second features comprises
air holes.

8. The optical fiber of claim 6, wherein at least one of the plurality of
webs is substantially radial and has a radial length and a transverse
width, the radial length greater than the transverse width.

9. The optical fiber of claim 2, wherein the spectral bandwidth is at
least about 200 nm.

10. The optical fiber of claim 2, wherein the spectral bandwidth is at
least about 1 μm.

11. The optical fiber of claim 2, wherein the spectral bandwidth is in a
range from about 50 nm to about 1 μm.

12. The optical fiber of claim 2, wherein the optical fiber has a
dispersion tailored to provide supercontinuum generation in the nonlinear
optical device.

13. The optical fiber of claim 2, wherein the optical fiber is configured
to control dispersion of the light and substantially confine the light to
the core.

14. The optical fiber of claim 2, wherein at least a portion of the fiber
is doped with a gain medium.

15. The optical fiber of claim 14, wherein the fiber is configured to
provide non-linear amplification of light propagating in the fiber.

16. The optical fiber of claim 14, wherein the fiber is configured to
receive optical pump light to pump the gain medium at a pump wavelength.

17. A frequency comb source comprising the optical fiber of claim 2.

18. A supercontinuum source comprising the optical fiber of claim 2.

19. An optical parametric amplifier system comprising the optical fiber
of claim

20. A chirped pulse amplification system configured to produce ultrashort
laser pulses, the system comprising at least one of (i) a non-linear
fiber amplifier comprising the optical fiber of claim 2, at least a
portion of the fiber being doped with a gain medium, or (ii) the optical
fiber according claim 2, the optical fiber being undoped.

21. A non-linear fiber optic system configured to produce broadband
optical pulses, the system comprising: a laser source configured to
produce optical pulses having the wavelength; the optical fiber of claim
2 optically coupled to the laser source, wherein at least a portion of
the optical fiber is doped so as to provide an optical gain medium for
non-linear amplification of the optical pulses; and an optical pump
configured to pump the optical gain medium at a pump wavelength.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation of U.S. patent
application Ser. No. 12/407,663, filed Mar. 19, 2009, entitled "ULTRA
SMALL CORE FIBER WITH DISPERSION TAILORING," which claims priority to
U.S. Provisional Patent Application No. 61/039,717, filed Mar. 26, 2008,
entitled "ULTRA SMALL CORE FIBER WITH DISPERSION TAILORING;" each of the
foregoing applications is hereby incorporated by reference herein in its
entirety.

[0005] Ultra small core fibers ("USCFs") have a variety of applications
especially in devices that utilize optical nonlinearities. Ultra small
core fibers have been used for supercontinuum generation, wavelength
conversion, soliton-based pulse compression, and so forth.

[0006] U.S. Pat. No. 6,792,188 discloses a design where an inner layer of
small holes is used to achieve tailored dispersion of a photonic crystal
fiber. In this disclosure, a large number of air holes are used beyond
the inner layer of small air holes. A significant drawback of this design
is the need to use a large number of air holes beyond the inner layer of
air holes to reduce confinement loss, especially for core diameters less
than 2 μm.

[0007] In a paper by J. K. Ranka, et al., "Optical Properties of
High-Delta Air Silica Microstructure Optical Fibers," Optics Letters,
vol. 25, pp 796-798, 2000, the authors disclose a fiber with a core
diameter of 1.7 μm surrounded by a triangular arrangement of a large
number of air holes with d/Λ≈0.9, where d is the diameter
of an air hole and Λ is the center-to-center spacing of the air
holes. The need for low confinement loss leads to the large air hole size
(relative to the hole spacing) and the large number of air holes. The
need for low confinement loss makes dispersion tailoring very difficult
for a fixed core diameter.

[0008] U.S. Pat. No. 7,266,275 discloses a method of dispersion tailoring
for a fiber incorporating a partially doped core to raise its refractive
index. For small core diameters less than 2 μm, glass and air boundary
plays a very significant part in confining light in the core. A
refractive index change of a few percent over part of the core has very
little impact on fiber dispersion.

[0010] Various embodiments include an optical fiber capable of propagating
light having a wavelength, the optical fiber comprising a core, a first
region at least partially surrounding the core, and a second region at
least partially surrounding the first region. The first region comprises
a plurality of first features. The first features have a first dimension,
and the plurality of first features have a first filling factor in the
first region. The second region comprises a plurality of second features.
The second features have a second dimension and the plurality of second
features have a second filling factor in the second region. The first
dimension is less than the second dimension and the second filling factor
is greater than about 90 percent.

[0011] Various embodiments include an optical fiber capable of propagating
light having a wavelength wherein the optical fiber comprises a core, a
first region at least partially surrounding the core, an air cladding
surrounding the first region, an outer layer surrounding the air
cladding, and a plurality of webs mechanically coupling the first region
and the outer layer such that the air cladding is disposed therebetween.
The first region comprises a plurality of first features. The first
features have a first dimension and the plurality of first features have
a first filling factor in the first region. The air cladding has an
air-filling factor greater than about 90%.

[0012] Various embodiments include an optical fiber capable of propagating
light having a wavelength, wherein the optical fiber comprises a core, a
first air cladding at least partially surrounding the core and a second
air cladding at least partially surrounding the first air cladding. The
first air cladding comprises a plurality of air holes having a first
size. The second air cladding comprises a plurality of air holes having a
second size. The second size is greater than the first size. The first
air cladding and the second air cladding are configured so that the fiber
dispersion has a zero dispersion wavelength less than the wavelength of
the light.

[0013] Various embodiments include a non-linear fiber optic system for
producing broadband optical pulses comprising a laser source producing
optical pulses having a wavelength, an optical fiber optically coupled to
the laser source and capable of propagating light having said wavelength,
and means for controlling dispersion of the pulses and for substantially
confining the pulse to the core. The optical fiber receives energy from
the laser source at a peak power. The optical fiber comprises a core
having a diameter less than about 4 μm and sufficiently small such
that the peak power exceeds a threshold for non-linearity of the optical
fiber. The fiber produces broadband amplified pulses having a spectral
bandwidth of at least about 50 nm.

[0014] For purposes of this summary, certain aspects, advantages, and
novel features are described. It is to be understood that not necessarily
all such advantages may be achieved in accordance with any particular
embodiment. Thus, for example, those skilled in the art will recognize
that embodiments may be provided or carried out in a manner that achieves
one advantage or group of advantages as taught herein without necessarily
achieving other advantages as may be taught or suggested herein.
Furthermore, embodiments may include several novel features, no single
one of which is solely responsible for the embodiment's desirable
attributes or which is essential to practicing the systems and methods
described herein. Additionally, in any method or process disclosed
herein, the acts or operations of the method or process may be performed
in any suitable sequence and are not necessarily limited to any
particular disclosed sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 schematically illustrates a cross-section of an embodiment
of an optical fiber. The fiber comprises a core with a diameter 2ρ,
and a cladding comprising holes with diameter, d, and center-to-center
separation, A.

[0016] FIGS. 1A and 1B provide examples of methods to estimate filling
fractions or factors for a finite matrix (or array) of cladding features
of regular and arbitrary shapes, respectively.

[0017] FIG. 2 is a graph that shows an example of simulated results for
dispersion of fibers having the cross-section schematically illustrated
in FIG. 1. In this example, the fiber cladding comprises six air holes
with a refractive index of 1 and with d/Λ=0.99. The graph shows
dispersion (in units of ps/nm/km) as a function of wavelength (in μm)
for various core diameters.

[0018] FIG. 3A schematically illustrates a cross-section of an embodiment
of a USCF comprising a core surrounded by first features that comprise
relatively small holes. The core and the first features are surrounded by
second features that comprise relatively larger holes. In some
embodiments, the first features may be used to tailor dispersion and the
second features may be used to provide optical confinement.

[0019] FIG. 3B schematically illustrates a length of an embodiment of a
USCF that is spliced to a conventional fiber. This embodiment of the USCF
provides low loss splice due to expansion of an optical mode at the
splice.

[0020] FIG. 4 is a graph that shows an example of simulated results for
dispersion of an embodiment of a USCF fiber having a cross-section as
shown in FIG. 3A. In this simulation, the USCF has a core diameter of 1.5
μm, and the first features are circular with diameter d and
center-to-center spacing Λ. The graph shows dispersion (in units
of ps/nm/km) as a function of wavelength (in μm) for various
d/Λ of the first features.

[0021] FIG. 5 is a graph that shows an example of simulated results for
dispersion of fibers having a core diameter of 1.25 μm. The graph
shows dispersion (in units of ps/nm/km) as a function of wavelength (in
μm) for various d/Λ of the inner cladding features.

[0022] FIG. 6 is a graph that shows an example of simulated results for
dispersion of fibers having a core diameter of 1.0 μm. The graph shows
dispersion (in units of ps/nm/km) as a function of wavelength (in μm)
for various d/Λ of the inner cladding features.

[0023] FIG. 7 schematically illustrates an embodiment of a preform stack
for making canes and a cross section of an embodiment of a fabricated
cane. The cane may be drawn into an embodiment of USCF.

[0024] FIG. 8 includes scanning electron microscope (SEM) photographs of
an embodiment of a USCF drawn using an embodiment of the cane shown in
FIG. 7. The left panel shows the fiber cross-section, and the right panel
is a closeup view of the center regions of the fiber.

[0025] FIG. 8A illustrates an example polarization maintaining fiber
oscillator-amplifier coupled to a highly nonlinear fiber in conjunction
with one embodiment of an oscillator phase control system.

[0026] FIG. 8B illustrates one embodiment of the polarization maintaining
fiber oscillator of FIG. 8A wherein the oscillator design allows for
phase control of the oscillator.

[0027] FIGS. 8C-8E illustrate some of the possible approaches for
controlling the beat signal related to the carrier envelope offset
frequencies associated with the system of FIG. 8A.

[0034] These and other features will now be described with reference to
the drawings summarized above. The drawings and the associated
descriptions are provided to illustrate embodiments of the invention and
not to limit the scope of the invention. Throughout the drawings,
reference numbers may be reused to indicate correspondence between
referenced elements. In addition, the first digit of each reference
number generally indicates the figure in which the element first appears.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0035] In the following detailed description, reference is made to
"filling fraction," also known as "void-filling fraction" or
"air-filling" fraction. In certain embodiments, the filling fraction
refers to a cross-sectional area occupied by certain features in a region
(such as, e.g., air holes in a cladding) divided by a total
cross-sectional area of the region. The term filing factor is also used
herein synonymously with filling fraction.

[0036] In certain such embodiments, the filling fraction may be determined
for periodic features in the region, where, for example, a basic unit
cell is repeated to fill the region. One possible expression for the
filling fraction for circular features having a diameter d and
center-to-center spacing (pitch) Λ in an infinite triangular
matrix was disclosed in U.S. Pat. No. 6,444,133, entitled "Method of
Making Photonic Bandgap Fibers" as the following formula:

Filling Fraction = π 2 3 ( d Λ ) 2
##EQU00001##

[0037] This formula for the filling fraction may be applied to, for
example, conventional photonic bandgap or photonic crystal fibers having
a very large number of features (such as holes). In many such fibers, at
least a portion of the holes are circular in shape.

[0038] For the purpose of illustrating the filling factor for various
embodiments, it is instructive to provide estimates for a fiber having a
limited number of features and/or a finite matrix of features. Features
having various size and shapes may be utilized in various embodiments,
for example, and the features may have regular and/or irregular shapes.
In some embodiments, estimates for the filling fraction may not be
obtainable in closed form, and may be determined using computational
numerical methods.

[0039] In certain fiber embodiments, the fiber cross-section comprises a
regular matrix of substantially identical features that are symmetrically
arranged. A first example of a fiber cross-section having a finite number
of features is shown in FIG. 1A. In this example, an annular region (or
band) 1510 between concentric circles 1501 and 1502 includes a matrix of
twelve features 1500. In this example, each feature 1500 of the matrix
has substantially the same cross-sectional shape and size and has a
cross-sectional area AF. The feature area AF may be, for
example, computed from predetermined geometric parameters describing the
shape and size, digitized using a graphic tool, and/or estimated with any
suitable computational methods and means. In the example shown in FIG.
1A, an inner radius R1 of the annular region 1510 corresponds to the
inscribed circle 1501, which is tangent to inner edges of the features
1500. An outer radius R2 of the annular region 1510 corresponds to
the circumscribed circle 1502, which is tangent to outer edges of the
features 1500. The filling fraction may be estimated by calculating the
ratio of the total area of the features 1500 to the area of the annular
region 1510. In the example shown in FIG. 1A, the filling fraction may be
determined to be 12AF/π(R22-R12).

[0040] In other embodiments, the fiber cross-section may comprise features
that are irregularly shaped and/or asymmetrically arranged. In a second
example schematically illustrated in FIG. 1B, the fiber cross-section
comprises features 1600 that are disposed in a region (or band) 1610
having an inner boundary 1601 and an outer boundary 1602. In this
example, each of the boundaries 1601, 1602 comprises tangent lines
linking adjacent features. The boundaries 1601, 1602 may include a
portion of the edge of a feature 1600 in some cases. As can be seen from
the example in FIG. 1B, the inner boundary 1601 comprises line segments.
In some cases, lines linking adjacent features intersect before reaching
an edge of the feature (see, e.g., the inner boundary near features
1604-A and 1604-B). If line segments from adjacent features intersect the
edge of a feature before intersecting each other, then a portion of the
edge of the feature will form part of the boundary (see, e.g., feature
1605 where edge portion 1605-A forms part of the boundary 1602).

[0041] FIG. 1B demonstrates that portions of the boundaries of the region
1610 may be convex or concave. For example, in FIG. 1B, the boundary 1601
is convex and a portion 1606 of the boundary 1602 is concave. As can be
seen from FIG. 1B, a boundary may not intersect all the features. For
example, the inner boundary 1601 does not intersect the features 1604-A
or 1604-B. Note that in both these embodiments, for a given portion of
the region (or band) 1610 between two features, the region (or band) is
at least as thick as the smallest of the two features. Nowhere is the
region or band 1610 thinner than the two closest features that define the
region or band.

[0042] The above-described methods for determining the filling fraction
may be used for fiber embodiments having cross-sections with multiple
regions of features. Estimates obtainable using mathematical formulas,
numerical computations (including manual estimates) are sufficiently
accurate so as to not substantially affect predicted optical propagation
properties of embodiments of USCFs.

[0043] Three problems have hampered practical development of state-of-art
USCFs. The first problem is the difficulty in splicing a USCF to a
conventional fiber due to the large mode size mismatch of the USCF and
the conventional fibers. For example, a USCF can have a mode field
diameter (MFD) of less than 3 μm, while conventional fibers typically
have MFD larger than 6 μm. The second problem is the difficulty of
tailoring fiber dispersion while maintaining a low confinement loss. Many
USCF fibers have a cladding formed by air holes in a background material,
typically a glass. For very small core diameter optical fibers, very
large air holes are required to reduce or minimize confinement loss
and/or to avoid having an excessively large number of air holes. This
leads to inflexibility for tailoring dispersion of the USCF. Dispersion
tailoring is advantageous for optimized operation of certain devices
utilizing optical nonlinearities, because of the ability to phase match
and/or group velocity match at different wavelengths and/or to operate in
higher order soliton modes. The third problem confronting a USCF is high
loss. USCF loss arises primarily from scattering loss at glass-air
interfaces. High loss occurs for two reasons when the core diameter is
small. The first reason is that there is much more optical energy at the
glass-air interfaces for small core fibers than for large core fibers.
Certain USCF comprise thin glass webs, which result from using large air
holes to reduce confinement loss. The second reason for the scattering
loss is that the webs tend to have more surface irregularities due to
their small thickness, which leads to more scattering loss.

[0044] Computer simulations have been performed to calculate the
dispersion in optical fibers. As described above, in some small core
fibers with small core diameter (<2 μm), very large air holes are
used to reduce or minimize confinement loss. To calculate the dispersion
of such fibers, the computer simulation uses a fiber design having a
cross-section shown in FIG. 1. The fiber of FIG. 1 comprises a cladding
having six large circular holes 1001 having a diameter of d. The
center-to-center separation of the holes 1001 is Λ. For the
simulation, the holes 1001 are assumed to be filled with a material
having a refractive index of one (e.g., air). The fiber comprises a core
1002 having a diameter 2ρ=2Λ-d.

[0045] Because high optical nonlinearity is achievable with very small
core sizes, various embodiments utilize very small core fibers, for
example, a fiber having a core of with a diameter in a range from about 1
μm to about 4 μm. In certain applications, it may be desirable to
achieve a relatively high index contrast between the holes 1001 and the
surrounding material in order to guide an optical beam propagating within
the fiber. Therefore, in certain preferred embodiments the holes 1001 are
considered to be filled with a gas or a mixture of gases (e.g., air), to
provide a reasonably high index contrast.

[0046] FIG. 2 is a graph that shows an example of simulated results for
the dispersion of fibers having the cross-section schematically
illustrated in FIG. 1. The graph shows dispersion (in units of ps/nm/km)
of the fiber as a function of the wavelength (in μm) of light
propagating in the fiber. The wavelength range shown in FIG. 1 is from
0.5 μm to 2.0 μm. In this example, the simulation was performed for
d/Λ=0.99. Curves 2001-2008 illustrate the dispersion for eight
values of the core diameter 2ρ=0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, and
2.0 μm, respectively. The dispersion curves 2001-2008 show few
features in this simulation. A zero dispersion wavelength (ZDW) is a
wavelength at which the dispersion is zero. The dispersion curves
2001-2008 each exhibit two ZDWs, the first ZDW at a smaller wavelength
than the second ZDW (the second ZDW is not shown in FIG. 2 for curves
2006, 2007, and 2008). FIG. 2 demonstrates that as the core diameter
decreases, the first and the second ZDW move toward shorter wavelengths.
For wavelengths between the first and the second ZDW, the dispersion is
positive (e.g., anomalous dispersion). FIG. 2 demonstrates that the
maximum of the anomalous dispersion is about 200 ps/nm/km for all fiber
core diameters used in the simulation, with the exception of the fiber
with the 0.6 μm core diameter (the curve 2001).

[0047] For a variety of nonlinear devices providing, for example,
supercontinuum generation and/or wavelength conversion, low dispersion
and/or relatively flat dispersion may be advantageous, due to the need
for higher-order soliton effects and phase matching among different
optical wavelengths in some such devices. Additionally, a small core is
generally desirable in these fibers, because enhanced nonlinearity can be
achieved through higher optical intensity in a small core fiber at a
fixed optical power. The results shown in FIG. 2 demonstrate that small
core fibers having the cross-section shown in FIG. 1 generally do not
provide low and/or relatively flat dispersion. Accordingly, a
disadvantage of such USCFs is the inability to simultaneously satisfy the
need for both low confinement loss and tailored dispersion without having
to use an excessive large number of air holes.

[0048] FIG. 3A schematically illustrates an example cross-section of an
embodiment of a USCF 3001. The USCF 3001 comprises a core 3002, a first
region comprising first features 3003, and a second region comprising
second features 3004. The first region substantially surrounds the core
3002, and the second region substantially surrounds the first region. An
outer layer 3005 substantially surrounds the second region. The core 3002
and the outer layer 3005 may be formed from the same material or from
different materials. In some embodiments, the core 3002 and/or the outer
cladding 3005 comprise a glass such as, for example, fused silica. In
some embodiments, the core 3001 comprises fused silica doped with one or
a combination of germanium, phosphorous, fluorine, boron, aluminum,
titanium, tin, and rare earth elements. For example, in the embodiment
illustrated in FIG. 3A, the core 3002 of the fiber 3001 comprises a core
region 3006 that is doped with a dopant such as, e.g., germanium. The
core region 3006 may form a central portion of the core 3002 as depicted
in FIG. 3A. Embodiments of the fiber 3001 comprising a doped core region
3006 may provide enhanced optical nonlinearity and/or reduced splice
loss, for example, as described in U.S. patent application Ser. No.
11/691,986, filed Mar. 27, 2007, entitled "Ultra High Numerical Aperture
Optical Fibers," which is owned by the assignee of the present
application, and which is hereby incorporated by reference herein in its
entirety. In other embodiments, the glass may comprise an oxide glass, a
fluoride glass, and/or a chalcogenide glass, any of which may be doped
with one or more of dopants described above for fused silica. In certain
embodiments, at least a portion of the core 3001 is doped to provide
optical gain. In some embodiments, the fiber 3001 may be "all-glass,"
such that the core 3002 and the outer layer 3005 in which the first and
the second features 3003, 3004 are disposed may comprises glass at least
over a portion of the fiber length. In some all-glass embodiments, the
first features 3003 and/or the second features 3004 may comprise a glass.

[0049] In the embodiment illustrated in FIG. 3A, the first features 3003
and the second features 3004 comprise holes that may be at least filled
with a material having a refractive index different from the material
forming the core 3002 and/or the outer layer 3005. For example, in some
embodiments, the first and the second features 3003, 3004 are filled with
air. In other embodiments, some or all of the first and/or the second
features 3003, 3004 may be filled with other gases and/or liquids.
Generally, a refractive index of approximately unity for the material in
the features is desirable for increased index contrast. In yet other
embodiments, some or all of the first and/or the second features 3003,
3004 are evacuated to provide a partial vacuum.

[0050] In the embodiment of the fiber 3001 schematically illustrated in
FIG. 3A, the first features 3003 comprise six substantially circular
holes that are arranged substantially symmetrically around the core 3002.
In other embodiments, a different number of first features may be
utilized such as, for example, 1, 2, 3, 4, 5, 10, or more features. In
other embodiments, the first features 3003 may be arranged differently
than shown in FIG. 3A and/or may have different shapes (e.g.,
non-circular) than shown in FIG. 3A. For example, the first features 3003
are not symmetrically arranged around the core 3002 in some embodiments.

[0051] In the embodiment of the fiber 3001 schematically illustrated in
FIG. 3A, the second features 3004 comprise twelve holes that are arranged
substantially symmetrically around the core 3002 and around the first
features 3003. In other embodiments, a different number of second
features may be utilized such as, for example, 1, 2, 3, 4, 5, 10, 15, 24,
or more features. In other embodiments, the second features 3004 may be
arranged differently than shown in FIG. 3A. For example, the second
features 3004 are not symmetrically arranged around the core 3002 and/or
the first features 3003 in some embodiments. In the embodiment shown in
FIG. 3A, the second features 3004 comprise radially elongated holes
having a "teardrop" shape. Other shapes are used in other embodiments. In
some embodiments, adjacent second features 3004 are disposed relatively
close to each other, thereby forming a relatively thin, elongated web
3010 between the adjacent features. For example, in the embodiment shown
in FIG. 3A, the fiber 3001 comprises 12 webs 3010. The ratio of radial
length to transverse width may be equal to or greater than about 4, about
6, about 8, about 10 or more in some embodiments.

[0052] In the embodiment of the fiber 3001 depicted in FIG. 3A, the first
features 3003 have a diameter A. The second features 3004 have a radial
length R and an azimuthal length L. In the illustrated embodiment, the
diameter A of the first features 3003 is less than the radial length R
and the azimuthal length L of the second features 3004. The radial length
is measured along the centerline through second features 3004 in the
radial direction, and the azimuthal (e.g. arc) length L is measured
through the azimuthally directed line midway along the radial length of
the second features 3004. For example, in some embodiments, the ratio R/A
(and/or the ratio L/A) may be in a range from about 1 to about 50. In
certain embodiments, the ratio of R/A (and/or L/A) is equal to or greater
than about 1, about 2, about 3, about 4, or about 5. Additionally, the
ratio R/L is equal to or greater than about 2. Different ratios are used
in other embodiments. As discussed above, first region comprises the
first features 3003, and the second region comprises the second features
3004. In certain embodiments, the first features 3003 have a first
filling fraction in the first region, and the second features 3004 have a
second filling fraction in the second region. The first filling fraction
and the second filling fraction may have any suitable values. For
example, in certain embodiments the first filling fraction is between
about 0.2% and about 90%. The first filling fraction is greater than
about 30% in some embodiments. In other embodiments, the first filling
factor is greater than about 5%, greater than about 15%, greater than
about 25%, greater than about 35%, greater than about 45%, greater than
about 55%, or some other value. In certain embodiments the second filling
fraction is between about 90% and about 99.9%. The second filling
fraction is greater than about 90% in some embodiments. In other
embodiments, the second filling factor is greater than about 50%, greater
than about 60%, greater than about 70%, greater than about 80%, greater
than about 95%, or some other value. For example, in the embodiment of
the USCF 3001 schematically illustrated in FIG. 3B, the first filling
factor is about 55% and the second filling factor is about 95%.

[0053] In some embodiments of the fiber 3001, the first features 3003 may
be used to tailor dispersion, and the second features 3004 may be used to
provide optical confinement for light propagating in the fiber 3001
(e.g., to reduce confinement loss). For example, in certain embodiments,
the size of the first features 3003 may be used for dispersion tailoring,
and the radial and/or azimuthal size of the second features 3004 may be
used for reducing confinement loss.

[0054] In some embodiments, the core 3002 may have a size that is about
one-half the wavelength λ of the light propagating in the fiber
3001. Fiber embodiments with core sizes as small as λ/2 may provide
reasonable confinement loss and a range of tailored dispersion. An
advantage of some embodiments of the fiber 3001 is that confinement of
optical power by the first features 3003 reduces the amount of optical
power at the interfaces of the second features 3004 (e.g., air-glass
interfaces in some embodiments). As described above, in certain
embodiments, the arrangement of the second features 3004 may form a
plurality of webs 3010. In certain such embodiments, the webs 3010 may
have a relatively high surface area that may include surface
irregularities, which could possibly contribute toward a higher
scattering loss. An advantage of some embodiments of the fiber 3001 is
that the reduction of optical power (by the first features 3003) in the
region of the second features 3004 also may reduce scattering losses at
the web interfaces, thereby effectively reducing the scattering loss of
the fiber 3001.

[0055] Another possible benefit of some embodiments of the fiber 3001 is
that the fiber 3001 may be spliced to a conventional fiber with
relatively low splice loss. The conventional fiber may a step-index
fiber, a graded-index fiber, or any other suitable optical fiber. In some
embodiments, the fiber 3001 may be spliced to a holey fiber, photonic
crystal fiber, or a length of fiber that is substantially similar to the
fiber 3001.

[0056] FIG. 3B schematically illustrates an embodiment of the fiber 3001
coupled at a splice 3200 to a conventional fiber 3100. In this
illustrative example, the conventional fiber 3100 is a high numerical
aperture fiber comprising a core 3101 that is larger than the core 3002
of the fiber 3001. The fiber 3100 supports propagation of an optical mode
3102. While propagating in the fiber 3100, the optical mode 3102 is
confined substantially to the core as indicated by the curve 3102 in FIG.
3B, which schematically represents a modal energy distribution.

[0057] The splice 3200 may be produced by any suitable splicing technique
such as, for example, a fusion splice. For example, in one embodiment of
a method for splicing the fiber 3001 to the fiber 3100, a section of the
fiber 3001 is heated (e.g., by an electric arc) before splicing in order
to reduce or substantially eliminate the first features 3003. After
heating, the section may have a reduced cross-sectional area in some
embodiments. In certain embodiments of this method, some of the heated
section of the fiber 3001 fuses, melts, or collapses into a substantially
solid structure with substantially total elimination of the first
features 3003 (possibly due to the larger surface tension of small air
holes in some embodiments). In certain embodiments, the second features
3004 are reduced in size by the heating, but the second features near the
splice region (indicated by reference numeral 3202) are not fully
collapsed (see, FIG. 3B). In other embodiments, the second features 3004
are substantially fully collapsed near the splice 3200. In some
embodiments of the splicing method, the core 3002 of the USCF 3001
remains substantially intact during the heating/collapsing/splicing
process. For example, in the embodiment illustrated in FIG. 3B, the
optional doped region 3006 of the core 3002 remains intact indicated by
reference numeral 3201) near the splice 3200. The optional doped region
3201 near the splice 3200 may further confine optical power in the core
3201 and lower splice loss at the splice 3200.

[0058] FIG. 3B schematically illustrates propagation of an optical mode
3007 in the USCF 3001 toward the splice 3200. While propagating in the
USCF 3001, the mode 3007 is confined substantially to the core 3001. As
the optical mode 3007 nears the region where the splicing process has
reduced (or substantially eliminated) the first features 3003, the mode
3007 begins to expand as schematically illustrated in FIG. 3B. In certain
embodiments, expansion of the mode 3007 may cause negligible loss if an
adiabatic condition is satisfied near the splice 3200, e.g., if the
reduction of the first features 3003 occurs more slowly than what a local
optical mode can follow. In the illustrated embodiment, when the mode
3007 reaches the splice 3200, the optical power of the mode 3007
substantially matches the optical power of the mode 3102 that can be
propagated in the core of the conventional fiber 3100. For example, the
above-described method advantageously permits a low loss splice to be
performed between the USCF 3001 and the conventional optical fiber 3100,
due to the substantially close match of the mode field diameters at the
splice 3200.

[0059] FIG. 4 is a graph that shows an example of simulated results for
dispersion of an embodiment of a USCF fiber having the cross-section
shown in FIG. 3A. In this simulation, the USCF has a core diameter
2ρ=1.5 μm, and the first features are circular with diameter, d,
and center-to-center spacing, Λ. The graph shows dispersion (in
units of ps/nm/km) as a function of wavelength (in μm) for various
d/Λ. The wavelength range in FIG. 4 is between 0.6 μm and 1.6
μm. Curves 4001-4009 are for d/Λ=0.9, 0.8, 0.7, 0.6, 0.5, 0.4,
0.3, 0.2, and 0.1, respectively. The results shown in FIG. 4 demonstrate
that a range of dispersion may be achieved for this embodiment of a USCF
having a core diameter of 1.5 μm. For example, a relatively flat, low
dispersion can be achieved near a wavelength of 1 μm for USCF
embodiments having d/Λ between about 0.4 and about 0.6. The
dispersion curve 4004 has two zero dispersion wavelengths (ZDW), a first
ZDW at a shorter wavelength (about 0.75 μm), and a second ZDW at a
longer wavelength (about 1.56 μm). Curves 4001, 4002, 4003, 4005,
4006, 4007, 4008, and 4009 have a single ZDW in the simulated wavelength
range shown in FIG. 4. In some nonlinear systems such as, for example,
systems providing supercontinuum generation, it may be advantageous to
pump the system at the anomalous dispersion side of the longer wavelength
ZDW. Because many convenient pump sources emit light at wavelengths of
about 1.05 μm, it may be beneficial to tailor the dispersion of the
USCF so that the first ZDW occurs at wavelengths slightly shorter than
about 1.05 μm. FIG. 4 demonstrates that the dispersion of embodiments
of the USCF disclosed herein may be tailored so that the first ZDW occurs
in a range from about 0.7 μm to about 0.87 μm, and such USCF
embodiments may be advantageously used in systems providing
supercontinuum generation.

[0060] FIG. 5 is a graph that shows another example of simulated results
for dispersion of an embodiment of a USCF fiber having the cross-section
shown in FIG. 3A. In this example, the USCF has a core diameter
2ρ=1.25 μm, and the graph shows the dispersion for three values of
d/Λ. Curves 5001, 5002, and 5003 are for USCF embodiments having
d/Λ=0.8, 0.6, and 0.4, respectively. FIG. 5 shows for these USCF
embodiments that a relatively flat dispersion may be provided, and the
dispersion may be tailored so that the zero dispersion wavelengths occur
in desired wavelength regions. For example, the first ZDW for the curve
5003 is at about 0.94 microns, and the dispersion is relatively flat
between about 1 micron and about 1.25 microns.

[0061] FIG. 6 is a graph that shows another example of simulated results
for dispersion of an embodiment of a USCF fiber having the cross-section
shown in FIG. 3A. In this example, the USCF has a core diameter
2ρ=1.0 μm, and the graph shows the dispersion for eight values of
d/Λ. Curves 6001-6008 are for d/Λ=0.9, 0.8, 0.7, 0.6, 0.5,
0.4, 0.3, and 0.2, respectively. FIG. 6 demonstrates that certain
embodiments of the USCF may provide tailored positive dispersion,
negative dispersion, and/or flattened dispersion in various portions of
the wavelength range shown in the graph.

[0062] In certain implementations, embodiments of USCF may be fabricated
according to a method in which a preform stack is formed into a cane and
the cane is drawn into an optical fiber. FIG. 7 schematically illustrates
an embodiment of a preform stack for making canes and a cross-section of
an embodiment of a fabricated cane. In the example shown in FIG. 7, a rod
7001 having a diameter of about 1.19 mm was disposed substantially in the
center of a preform stack. The rod 7001 was solid and included a
germanium-doped silica (SiO2) center portion surrounded by a silica
cladding. Six tubes 7002 were disposed around the rod 7001. Each of the
tubes 7002 was hollow and had an inner diameter of about 0.61 mm and an
outer diameter of about 1.19 mm. Twelve tubes 7003 were disposed around
the tubes 7002 as shown in FIG. 7. Each of the tubes 7003 was hollow had
an inner diameter of about 0.76 mm and an outer diameter of about 1.19
mm. In other embodiments, some or all of the tubes 7002, 7003 may be
partially or completely solid.

[0063] The stack of the rod 7001, the tubes 7002, 7003 was disposed in an
outer tube 7004. In this example arrangement, the outer tube 7004
comprised silica and had an inside diameter of about 6.2 mm and an
outside diameter of about 8.4 mm. In this example, a pressurizing system
was use to apply a pressure of about 2 psi to the inside of the tubes
7002 and 7003 during the caning process. While the tubes were
pressurized, a cane 7100 having a 1.63 mm outer diameter was drawn. In
certain embodiments, one or more inert gases are used in the
pressurization system, while in other embodiments, air, nitrogen, oxygen,
and/or other gases can be used.

[0064] The cane 7100 was inserted into an outer tube having an inner
diameter of about 2 mm and an outer diameter of about 16.33 mm. In this
example fabrication process, a pressure of about 2 psi was applied to the
hollow tubes, and a partial vacuum of about -5 in Hg was applied to the
inside of the outer tube. While pressurized, the preform was drawn at a
temperature of about 195° C. The preform was fed into the heating
furnace at about 6 mm/min and was drawn at 92 mm/min into a fiber with an
outer diameter of about 125

[0065] FIG. 8 includes scanning electron microscope (SEM) photographs of
an embodiment of USCF 8001 drawn with the cane 7100 shown in FIG. 7. The
left panel of FIG. 8 shows the cross section of the fiber 8001, and the
right panel is a closeup view showing the inner structure 8002 of the
fiber 8001, which comprises a core 8002, first features 8003, and second
features 8004. In this embodiment of the fiber 8001, the first features
8003 comprise six substantially circular holes, and the second features
8004 comprise twelve "teardrop" shaped holes. FIG. 8 demonstrates that in
this example embodiment, the second features 8004 are significantly
expanded under pressure during fiber drawing to achieve a larger
air-filing factor than possible with certain conventional stack-and-draw
processes such as, for example, the process described in U.S. Pat. No.
6,792,188.

[0066] This fabrication procedure described herein is one possible
embodiment of the fabrication procedure. In other embodiments of the
procedure, variations of rod and tube dimensions, applied pressure,
applied vacuum, and drawing condition may be used. In other embodiments,
additional layers of tubes may be used to provide third features, fourth
features, and so forth. Different numbers of tubes may be used. Further
details of fiber fabrication procedures are described in, for example,
the above-incorporated U.S. patent application Ser. No. 11/691,986.

Example Applications for USCF Embodiments

[0067] Embodiments of the USCFs described herein advantageously may be
utilized in a variety of applications including, for example, nonlinear
amplifiers, continuum generation, frequency metrology systems employing
comb generators, and systems for stretching ultrashort pulses.

[0068] The disclosure of U.S. patent application Ser. No. 11/372,859,
entitled "Pulsed Laser Sources," filed Mar. 10, 2006 (hereinafter the
'859 application), and published as U.S. Patent Application Publication
2006/0198398 is hereby incorporated by reference in its entirety. FIGS.
8A-8G (reproduced herein as FIGS. 8A-8G) and the corresponding text of
the '859 application describe embodiments of systems for frequency comb
generation, which may be used for frequency metrology and/or high
resolution spectroscopy. Embodiments of such systems may be useable for
precision timing measurements and/or for high resolution spectroscopy,
the latter application showing promise for disease detection.

[0069] By way of example, FIG. 8A of the '859 application illustrates an
embodiment of a polarization maintaining fiber oscillator-amplifier
coupled to a highly nonlinear fiber in conjunction with one embodiment of
an oscillator phase control system. In FIG. 8A, the frequency comb source
800 comprises a polarization maintaining fiber oscillator 801. An output
from the oscillator 801 is directed via an isolator 802 to a polarization
maintaining fiber amplifier 803. As shown in FIG. 8A, the fiber amplifier
803 is connected via a splice 804 to a highly nonlinear fiber (HNLF) 805.
The highly nonlinear fiber 805 is preferably constructed from a holey
fiber or a standard silica fiber or using bismuth-oxide based optical
glass fiber in various embodiments. The dispersion of the highly
nonlinear fiber 805 is preferably close to approximately zero at the
emission wavelength of the oscillator 801 for certain designs. Even more
preferably, the dispersion profile is flattened, i.e., the third-order
dispersion of the fiber 805 is equally close to approximately zero. The
highly nonlinear fiber 805 does not need to be polarization maintaining
since it is relatively short (on the order of few cm long), thereby
enabling long-term polarization stable operation. The length of the
highly nonlinear fiber 805 is preferably selected to be less than
approximately 20 cm to preserve the coherence of the generated continuum.
Other designs, however, are possible. Embodiments of the USCF may be
utilized in embodiments of the frequency comb source shown in FIG. 8A and
may, in various embodiments, extend the continuum, reduce pump
requirements, and/or provide for higher degree of coherence.

[0070] The (continuum) output from the highly nonlinear fiber 805 is
injected via a splice 806 to a wavelength division multiplexing coupler
807. The coupler 807 directs the long and short wavelength components
from the continuum to a long wavelength coupler arm 808 and a short
wavelength coupler arm 809 respectively. The long wavelength components
are subsequently frequency doubled using exemplary lenses 810, 811, 812,
813, as well as a doubling crystal 814. After frequency doubling the
resulting output preferably has a substantially same wavelength as at
least part of the short wavelength components traveling in the arm 809.
Additional optical elements 815 and 816 can be inserted into the beam
paths of the arms 808 and 809 for spectral filtering, optical delay
adjustment, as well as polarization control. Spectral filtering elements
are selected to maximize the spectral overlap of the signals propagating
in arms 808 and 809. As another example, the optical element 815 can
comprise appropriate wave-plates that control the polarization state of
the light in front of the doubling crystal 814.

[0071] The frequency-doubled light from the arm 808 and the light from the
arm 809 are subsequently combined in a polarization-maintaining coupler
817 which preferably has a 50/50 splitting ratio. The beat signal from
interference of the two beams injected into the coupler 817 is detected
by a detector 818.

[0072] As shown in FIG. 8A, one selected harmonic of the beat signal at
frequency f.sub.n,m,beat may be directed via an electrical feedback
circuitry 819 to the oscillator 801.

[0073] An optical element 816a may be inserted in an optical path after
the two arms 808, 809 are combined. The optical elements 816 and 816a
that can be inserted into the arm 809 and in the combined signal arm
before the detector 818 may comprise a narrow bandpass filter that
narrows the spectral width of the signal transmitted through the arm 809.

[0074] To produce an optical output of the frequency comb source which is
used, for example, for a frequency metrology experiment, part of the
frequency comb can be coupled out from a location 818b after the highly
nonlinear fiber or from a location 818a after the coupler 817 and
interferometer. The optical output can also be coupled out at a location
818d after the oscillator or at a location 818c after the amplifier, if
for example only the spectral part of the oscillator or amplifier
bandwidth of the comb is desired.

[0075] FIG. 8B illustrates one possible embodiment of the oscillator 801
described above in reference to FIG. 8A. The oscillator 801 includes a
saturable absorber module 820 comprising collimation and focusing lenses
821 and 822 respectively. The saturable absorber module 820 further
comprises a saturable absorber 823 that is preferably mounted onto a
first piezo-electric transducer 824. The first piezo-electric transducer
824 can be modulated to control, for example, the repetition rate of the
oscillator 801.

[0076] The oscillator 801 further comprises an oscillator fiber 825 that
is preferably coiled onto a second piezo-electric transducer 826. The
second piezo-electric transducer 826 can be modulated for repetition rate
control of the oscillator 801. The oscillator fiber 825 is preferably
polarization-maintaining and has a positive dispersion although the
designs should not be so limited. The dispersion of the oscillator cavity
can be compensated by a fiber grating 827 which preferably has a negative
dispersion and is also used for output coupling. It will be understood
that a positive dispersion fiber grating and a negative dispersion cavity
fiber may also be implemented. Furthermore, the fiber grating 827 can be
polarization-maintaining or non-polarization-maintaining.

[0077] The pump light for the oscillator 801 can be directed via a
polarization-maintaining wavelength division multiplexing coupler 828
from a coupler arm 829 attached to a preferably single-mode pump diode
830.

[0078] FIGS. 8C-D illustrate some of approaches to using the beat signal
frequency to control repetition rate as well as carrier envelope offset
frequency of the frequency comb source 800. As shown in FIG. 8C, a pump
current 840 can be changed, wherein a change in the pump current can
cause a change of the beat signal frequency and more particularly the
carrier envelope offset frequency.

[0079] As shown in FIGS. 8D and 8E, the absolute position of the carrier
envelope offset frequency can be controlled by adjusting the temperature
of the fiber grating 827 with a heating element 842. Alternatively,
pressure applied to the fiber grating 827 can also be used to set the
carrier envelope offset frequency using for example a piezo-electric
transducer 844.

[0080] FIGS. 8A and 8B describe the basic design of a frequency comb
source based on a low noise phase-locked fiber laser for frequency
metrology. Modifications to this basic design can be easily implemented
as described below.

[0081] FIG. 8F illustrates one embodiment of a fiber based continuum
source 850 where the amplifier (803 in FIG. 8A) is omitted. In the
exemplary continuum source 850, high quality sub-200 fs pulses are
preferably injected into a highly nonlinear fiber 854 (805 in FIG. 8A).
To generate such short pulses, the oscillator-only continuum source 850
preferably generates positively chirped pulses in the oscillator 801,
which are compressed in an appropriate length of a negative dispersion
fiber 852 before injection into the highly nonlinear fiber 854. For the
oscillator-only continuum source 850, the amplifier is thus substituted
with the negative dispersion fiber 852.

[0082] As shown in FIG. 8F, the oscillator-only continuum source 850
further comprises an interferometer 856 that interferes the two frequency
components as described above. The interferometer 856 may be similar to
the two-arm interferometer shown in FIG. 8A (fiber based or equivalent
bulk optics components), or may be similar to a one-arm interferometer
described below. The output of the interferometer 856 can be detected by
a detector 858, and selected signals from the detector 858 can be used
for feedback control 860 in a manner similar to that described above in
reference to FIGS. 8A-E.

[0083] FIG. 8G illustrates an example one-arm interferometer 870. Such an
interferometer can be obtained by removing one of the arms (arm 809 in
FIG. 8A) and modifying the remaining arm. As shown in FIG. 8G, the
interferometer 870 comprises a group delay compensator 872 inline with a
doubling crystal 874. The group delay compensator 872 receives a
continuum signal from a highly nonlinear fiber located upstream, and
ensures that the frequency doubled and non-doubled spectral components
from the continuum that are output from doubling crystal overlap in time.
Moreover, since the doubled and non-doubled spectral components are
selected to overlap in optical frequency, these components interfere and
the interference signal is detected with a detector downstream.

[0084] Highly non-linear fibers corresponding to embodiments of the
present disclosure may be utilized for supercontinuum generation and may
provide for extremely broad bandwidths. The disclosure of U.S. patent
application Ser. No. 11/091,015, entitled "Optical parametric
amplification, optical parametric generation, and optical pumping in
optical fibers systems," filed Mar. 25, 2005 (hereinafter the '019
application), and published as U.S. Patent Publication 2005/0238070 is
hereby incorporated by reference in its entirety. Embodiments may provide
a broad spectral bandwidth for continuum or supercontinuum generation.
For example, in various embodiments, the bandwidth may be at least about
50 nm, and in some embodiments at least about 200 nm. A spectral
bandwidth of up to about 1 μm may be generated with embodiments of
highly non-linear USCFs. For example, in some embodiments, supercontinuum
from about 0.4 μm to greater than about 1.6 μm may be generated.

[0085] FIG. 4 of the '015 application (reproduced herein as FIG. 9) and
the corresponding text of the '015 application, for example, describe an
embodiment of an amplification system comprising a short-pulse fiber
laser 101 whose output is split into two arms by a beam splitter 220. In
a one arm is an optical parametric amplification (OPA) pump 200 that
provides pump power. The OPA pump 200 outputs high-energy,
narrow-bandwidth, pump pulses. In another arm, a broadband continuum is
generated in a continuum fiber 210. This continuum fiber 210 may
comprise, for example, a fiber having nonlinear properties. Output from
the continuum fiber 210 is passed through a filter 240 to filter out
twice the center wavelength of the light generated by an OPA pump 200
located in a second arm. The filter 240 may pass long- and/or
short-wavelength parts relative to twice the center wavelength of the OPA
pump 200. This broadband continuum output comprises a seed pulse for
seeding the OPA process.

[0086] Accordingly, the output from the continuum fiber 210 after being
filtered by the filter 240 as well as the pump output from the OPA pump
200 are combined by a beamsplitter/coupler 250 and applied to the
parametric amplifier 260. The beam splitter 250 thus combines high-energy
narrow-bandwidth pump pulses from the OPA pump 200 and wide-bandwidth
seed pulses from the continuum fiber 210. An amplified signal is produced
by the parametric amplifier 260. This amplified signal is applied to the
pulse compressor 270.

[0087] The fiber laser 101 may be a mode-locked oscillator or a
mode-locked oscillator followed by one or more fiber amplifiers. The
fiber laser 101 is constructed to deliver pulse energies and peak powers
sufficient to produce a wide enough continuum in the continuum fiber 210,
e.g., a few nanojoules (nJ). For additional background, see, e.g., U.S.
Patent Publication 2004/0213302 entitled "Pulsed Laser Sources" filed by
Fermann et al, which is incorporated herein by reference in its entirety.
In various embodiments, the fiber laser 101 is an Er fiber laser that
produces short optical pulses at about 1560 nm with the repetition rate
of 20-100 megahertz (MHz). The laser 101 may produce linearly-polarized
light as for example can be obtained by using polarization-maintaining
(PM) components. The laser is optionally implemented as a
master-oscillator-power-amplifier (MOPA) configuration. Such lasers are
described in, e.g., U.S. Patent Application No. 60/519,447, which is
incorporated herein by reference in its entirety and available from IMRA
America, Ann Arbor Mich.

[0088] In the embodiment schematically illustrated in FIG. 9, the ultra
broadband continuum in one arm is generated in the continuum fiber 210,
which may comprise a micro-structured fiber and/or a conventional
solid-core high-nonlinearity fiber. Optionally, two or more different
nonlinear fiber types can be used sequentially as discussed in U.S.
Patent Publication 2004/0213302, which is incorporated herein by
reference in its entirety. With such an approach, continuum generation
can be optimized for different spectral parts, thereby resulting in
stable operation over a wide ultra broadband spectrum.

[0089] Alternatively, the output from the splitter 220 can be split into
two or more arms and different nonlinear fibers or sequences of nonlinear
fibers in different arms can be used to optimize the continuum output for
each individual arm. The optimization of the continuum output in each arm
is particularly useful when creating ultra broadband continua or
ultra-flat continua as well as low noise continua. Flat continua are
preferred in most applications to reduce or avoid the occurrence of
`spectral holes`. For example, in optical coherence tomography, spectral
holes limit the optical resolution. Equally, in spectroscopy, spectral
holes limit the signal/noise of a potential detection system in certain
parts of the spectrum, which is generally undesired.

[0090] Embodiments of the ultra-small core non-linear fibers disclosed
herein may be utilized in embodiments of supercontinuum generation
systems described in the '015 application, or in variations thereof.
Spectral widths of at least several hundred nm may be generated in some
embodiments.

[0091] The disclosures of U.S. patent application Ser. No. 10/437,059
entitled "Inexpensive variable rep-rate source for high-energy ultrafast
lasers," filed May 14, 2003 (hereinafter the '059 application), published
as U.S. Patent Application Publication 2004/0240037, and U.S. patent
application Ser. No. 10/813,163, entitled "Modular fiber-based chirped
pulse amplification system," filed Mar. 31, 2004 (hereinafter the '163
application), and published as U.S. Patent Application Publication
2005/0226286 generally relate to fiber based ultrashort systems. Various
embodiments include non-linear amplifiers for amplifying pulses and
configurations for pulse stretching and spectral broadening. The
disclosures of the '059 application and the '163 application are both
incorporated by reference herein in their entirety.

[0092] One application of embodiments of USCF is for stretching ultrashort
pulses. In some embodiments, it is desirable to stretch ultrashort pulses
to a pulse width of about 1 ns prior to amplification. The stretching may
be carried out in a fiber gain medium to provide both amplification and
spectral broadening, and/or in passive fibers. In certain embodiments,
fibers having normal dispersion (group velocity dispersion, GVD) are
utilized in combination with self-phase modulation to produce linear
chirped and broadened pulses at wavelengths at or near 1 μm.

[0093] A doped fiber gain medium provides for non-linear amplification. In
the '163 application at least one embodiment comprises a non-linear
amplifier module. It is nonlinear due to the fact the pulse is not
temporally stretched so that the amplification takes place with high
intensity and thus significant self-phase modulation. FIGS. 1B and 1C
from the '163 application are reproduced herein as FIGS. 10A and 10B.
Typical amplifier configurations are shown in FIG. 10A (a co-propagating
and counter-propagating pumped arrangement) and FIG. 10B (a side-pumped
arrangement), although the precise configuration can be selected from
many known amplifier designs. The spectrum at the output of this
amplifier is shown in FIG. 6A from the '163 application, which is
reproduced herein as FIG. 10C. For higher pump currents the spectral
width is over 20 nm. Thus in this nonlinear amplifier the spectral width
has been increased by self-phase modulation by more than a factor of 10,
from about 2 nm over 10 times to greater than 20 nm. The amplifier is a
Yb-doped cladding pumped fiber that is 4 meters long. In FIG. 10B, the
amplifier is side-pumped with counterpropagating pumping. Even at the
lowest current the spectrum has been broadened by self-phase modulation.
At the higher current levels, the spectrum is typical for self-phase
modulation propagating in a fiber with positive dispersion. Compare these
spectra to that shown in FIG. 7 of the '163 application, which is
reproduced from Govind P. Agrawal, Nonlinear Fiber Optics (Academic
Press, Inc. New York, 1989), and which is reproduced herein as FIG. 10D.

[0094] A nonlinear Yb amplifier with positive dispersion, usable with
amplifier embodiments such as those schematically shown in FIGS. 10A and
10B, and which has been utilized for pulse amplification of a
substantially unchirped pulse that is significantly spectrally broadened
during the amplification and which can be pulse compressed after
amplification is disclosed in the '059 application. In such systems,
highest gain and efficiency are not the predominant concern as in the
case of other amplifiers. The gain of about 100 times in this stage is
rather low for a fiber amplifier. One goal is to obtain the highest pulse
energy in a pulse that can be compressed.

[0095] Embodiments of the USCF disclosed herein may be used to further
improve the performance of the above-described ultrashort laser systems,
or similar systems.

[0096] Also, embodiments of the present disclosure may be utilized in a
wide-range of applications wherein one or more of femtosecond,
picosecond, nanosecond, and microsecond pulses are directed to a target
material. Further possible applications of both non-linear stretchers and
non-linear amplifiers (comprising USCF embodiments) are generally found
in material processing and micromachining operations. For example, a
material processing system may comprise a fiber system, an optical system
to direct the pulses to a material, at least one positioning system to
position the target material relative to one of more pulses, and a system
controller.

[0097] A wide variety of other applications, both currently known as well
as yet to be discovered, are also possible.

[0098] While certain embodiments of the disclosure have been described,
these embodiments have been presented by way of example only, and are not
intended to limit the scope of the present inventions. A wide variety of
alternative configurations are also possible.

[0099] For example, components (e.g., layers) may be added, removed, or
rearranged. Similarly, processing and method steps may be added, removed,
or reordered.

[0100] Accordingly, although certain preferred embodiments and examples
have been described above, it will be understood by those skilled in the
art that the present invention extends beyond the specifically disclosed
embodiments to other alternative embodiments and/or uses and obvious
modifications and equivalents thereof. In addition, while several
variations have been shown and described in detail, other modifications,
which are within the scope of this invention, will be readily apparent to
those of skill in the art based upon this disclosure. It is also
contemplated that various combinations or sub-combinations of the
specific features and aspects of the embodiments may be made and still
fall within the scope of the invention. It should be understood that
various features and aspects of the disclosed embodiments can be combined
with, or substituted for, one another in order to form varying modes and
embodiments. Thus, it is intended that the scope of the present invention
herein disclosed should not be limited by the particular disclosed
embodiments described above.